This invention relates to the production of very low packing density ceramic abrasive grits and methods of making the same from various known abrasive materials such as fused aluminum oxide, co-fused aluminum oxide-zirconium oxide or sintered sol gel aluminum oxide, all of which may or may not contain various supplementary oxides.
A usual abrasive particle can simplistically be represented by a double ended cone, whereby one end of the cone is bonded to the abrasive product base and the other end is contacting the steel workpiece. As the cone wears back from the tip of the cone, the wear flat is increasing in two directions and thus the wear flat area growth rate increases as a squared function of the wear back.
For use in abrasive applications, the above abrasive materials are produced in bulk form and are crushed to the desired grit size for use in coated or bonded abrasive products. Aluminum oxide abrasive can be made by refining bauxite in an arc furnace or by melting a Bayer type alumina in an arc furnace and pouring into large molds for cooling. The crude is then crushed into the desired grit size. Co-fused aluminum oxide zirconium oxide abrasive is also arc furnaced, but it is poured into molds which produce very rapid cooling. Such molds may be steel plates with small spacings (about ¼″) between them or molds which contain steel balls, whereby the melt flows into the interstitial spaces. This crude is then also crushed into desired grit sizes. Sol gel alumina is made in a water based system, usually extruded, dried, crushed into the desired grit sizes, calcined and fired into abrasive grit particles.
In all of the above, crushing of the bulk material results in particles with a three dimensional shape, whereby the dimensions of the particles in the three axes of the grain are equivalent or nearly equivalent. Abrasive grits for heavy duty bonded applications are desired to have equal axes which indicate a “blocky” material and is indicative of a material with a high packing density. Abrasive grits for paper and cloth applications are desired to have unequal axes which indicate a “sharp” material and is indicative of a material with a lower packing density. The table below illustrates typical packing densities of various bonded and coated abrasive grits.
TABLE IPacking Density 36 GritBondedCoatedAbrasivesAbrasivesg/cm3g/cm3Fused Aluminum Oxide1.82-1.921.73-1.82Sintered Sol Gel Aluminum Oxide1.82-1.921.73-1.82Co-Fused Aluminum Oxide-Zirconium Oxide2.10-2.221.99-2.10
The co-fused aluminum oxide-zirconium oxide particles have similar shapes to the fused aluminum oxide and sintered sol gel particles. The higher packing densities result from the higher true specific gravity of the alumina-zirconia (4.55) versus 3.94-3.92 for the fused aluminum and sol gel alumina.
Generally, the abrasive grits for coated applications are extracted during the initial crushing of the crude material when the abrasive particles are sharper as indicated by a lower packing density. The abrasive grits for some bonded applications are generally further processed by milling or impacting to make the particles more blocky as indicted by a higher packing density. In addition, some bonded applications do require lower packing density or sharper abrasive grits.
Of particular interest are the packing densities of the coated abrasive materials and that they are only slightly lower than the packing densities of the bonded abrasive materials. The coated abrasive packing densities are inherent from crushing a bulk material into abrasive grits. Lower packing density material than shown on the above Table I for coated abrasive applications is desirable but not attainable with the typical crushing of abrasive grits from bulk crude materials. Lower packing density abrasive may be associated with providing a longer life abrasive product in coated applications i.e. a higher total metal removal (cut) until removal rate is unacceptable.
An abrasive disc or belt is discarded when the grinding operator considers that it has become dull, which means the metal removal rate has decreased to approximately 10-20% of the initial metal removal rate. The metal removal rate is a function of the penetration of the abrasive particles into the steel workpiece. The penetration of the particles into the steel is further dependent on the pressure the abrasive grits apply to the steel workpiece. The pressure of the abrasive grits on the steel workpiece is defined by the force of the abrasive particles on the steel divided by the wear flat area of the particles. As the wear flat area of the abrasive particles increases, the abrasive grit pressure applied to the steel workpiece decreases, the abrasive grit penetration decreases and the resulting metal removal rate decreases. When a grinding disc or belt is discarded, the wear flat area is very small, 0.002 in.2 to 0.004 in.2 per square inch of abrasive material, depending on the type and shape of the steel being ground and the force applied to the abrasive products.
The wear flat area of an abrasive particle is dependent on the wear (wear back) of the particle during grinding. As the wear back increases, the wear flat area increases. To improve the useful cutting life of a coated abrasive product, it is necessary to slow the abrasive grit wear flat growth rate. This may be accomplished with an abrasive grain more resistant to wear which results in a slower wear back growth rate and a resulting slower wear flat growth rate. Secondly, the abrasive particle shape can be changed to result in a slower wear flat growth rate as the particle wears back. Thirdly, the shape and consistency of the abrasive particle can be altered so that the abrasive particles with terminal wear flat areas can shed the wear flats and expose a new grinding surface or that the particle can break off below the grinding interface thereby also removing the terminal wear flat. This shedding or break-off phenomenon can be called self dressing. The terminal wear flat area is defined as that area which prevents the abrasive particles from significantly penetrating the steel surface with a particular applied force.
Ceramic hollow spheres (ceramic bubbles) have been known for some time and have many applications, one of which is to lower the density of abrasive products, especially bonded abrasive products. Such bubbles, e.g. alumina or alumina-zirconia bubbles, added to reduce product density generally do not provide sharp edges or corners that significantly improve grinding performance; although, such bubbles may partially or completely fracture during use of the abrasive product which conceivably could have some such effect, albeit minor. Such ceramic bubbles are well known as are their methods of manufacture, e.g. by atomizing fused ceramic material, e.g. fused alumina or alumina-zirconia mix with compressed gas, usually compressed air. Such products are readily available in the industry, e.g. from Washington Mills as DURALAM® hollow ceramic spheres, from Zircar Corporation as ZIRCAR® hollow ceramic spheres or from Treibacher Corporation as ALODUR® hollow ceramic spheres.
Such ceramic bubbles may also be made from other processes such as a sol gel processes as described in U.S. Pat. No. 5,077,241.
It has also been known to introduce ceramic filaments or rods into abrasive products, e.g. as described in U.S. Pat. Nos. 5,194,072; 5,372,620; and 5,876,470 with idea that the end portions of such filaments or rods would retain a certain sharpness. While this may in some respects have validity, use in actual practice can be difficult. This is because to have the appropriate effect, the rods or filaments have to be appropriately oriented, i.e. a side on exposure of such a rod or filament to a work piece does not act as a good cutting edge. Further, the ends of such rods or filaments may not actually have particularly sharp cutting edges since they might be no thinner than the diameter of the rod or filament itself.